Justin Vernon also co-wrote the delicate ballad with Swift and an unknown person named William Bowery. Swift later revealed in her Disney+ concert film Folklore: The Long Pond Studio Sessions
. William Bowery is none other than her boyfriend, Joe Alwyn.
She explained that Alwyn wrote the "entire piano part" on "Exile" and originally sung the part of the duet that was eventually recorded by Bon Iver's Justin Vernon.
"He was just singing it, the way that the whole first verse is," she said. "So I was entranced and asked if we could keep writing that one. It was pretty obvious that it should be a duet because he's got such a low voice and it sounded really good sung down there in that register."
Swift has form for using pseudonyms (she was credited for her work on Calvin Harris' "This Is What You Came For
" under the name Nils Sjöberg). A possible clue for Alwyn's moniker is he attended a Kings of Leon after-party at New York's Bowery Hotel during the early days of their romance. Also, Joe is the great-grandson of composer, conductor, and music teacher William Alwyn.